I'm friendly to the personal message, simply stating, "I am not Christina, please update your address book."
If someone signs up for a website using my email, I immediately go there, reset their password, and change the email address to something like "abuse@yahoo.com."
If a kid signs up for an account with me as their parent, I immediately revoke their account. I'd want someone to do that to my kids if they were trying to get around me.
Any other accounts, I do what I can to alert the company, but for the most part they get labeled as spam. If the company did not take steps to verify the email address, then it is absolutely spam, and not my problem.
I have a 3 character yahoo ID, and I get a ton of these. Unfortunately I was never able to make any impact on the person filing tax returns using my email address.